The high incidence of osteoporosis in the elderly female population has been attributed to many factors including estrogen deficiency, low calcium intake and decreased calcium absorption. The importance of calcium bioavailability is uncertain because of the process of intestinal adaptation to low calcium availability. The magnitude, timing, and effects of aging on this adaptation need to be defined before calcium bioavailability can be studied effectively. Aims of this study include 1) determining the length of time necessary for stabilization of intestinal absorption after an acute decrease in calcium intake, 2) determining if the changes noted in 1) are associated with changes in levels of parathyroid hormone (PTH), 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)2D], or 24,25-dihydroxyvitamin D [24,25(OH)2D], and 3)\determining the effects of age on any changes noted in 1) and 2) above. Fifteen elderly (age 60 to 80) and fifteen young (age 20 to 35) healthy volunteers will be placed on self-selected diets plus supplementation to a total of 2 grams of calcium daily. After eight weeks their diet will be decreased to 300 mg of calcium daily. Four hundred IU vitamin D will be administered throughout the study to reduce variation in vitamin D intake. One week before and 1,2,4, and 8 weeks after the decrease in dietary calcium, the following parameters will be assessed: 25(OH)D, 1,25 (OH)2D, 24,25(OH)2D, PTH, serum calcium, and calcium absorption. The latter will be estimated by whole body counting (WBC), after ingestion of Ca47 and a nonabsorbable stool marker 51 Cr, as the sum of whole body retention plus urinary losses of 47C. Baseline bone density will be measured by dual photon absorptiometry.